Forgers beware. Faked signatures are much easier to detect now there is a system that can produce a three-dimensional image of a handwriting sample. The technique reveals extra details that can be compared with the genuine article.

Suspect signatures are usually analysed by expert graphologists, who compare the appearance of different letters in a name with a verified original. However, they are restricted to looking at flat, two-dimensional writing, and good forgeries can sometimes slip through the net.

The new three-dimensional analysis reveals the sequence in which each pen stroke was made on the page. The technique also highlights differences in the pressure applied by the writer as they marked the page. Such pressure differences are extremely difficult to mimic.

"It will be a powerful tool for forensic experts around the world," says Giuseppe Schirripa Spagnolo, leader of the team of physicists at the University of Rome Three, Italy, that developed the method. "We believe this is one of the most promising ways of detecting forged handwriting," he adds. The work is published in the latest edition of the Journal of Optics1.

Analysed to a 't'

Two signatures made by the same person are never quite identical. But the sequence of each pen stroke, and therefore the way that the lines overlap, will always be the same. For example, the cross of a 't' is normally always made after the upright is drawn, and generally moves from left to right.

“It will be a powerful tool for forensic experts around the world”

Giuseppe Schirripia SpagnoloUniversity of Rome Three, Rome

More subtle writing movements, which differ more between individuals, are also highlighted by the system. Such movements include whether a loop was drawn clockwise or anticlockwise.

As someone signs his or her name, the pen carves a microscopic furrow into the page. When one line crosses another it makes a slightly deeper depression in the paper, and also breaks down the walls of the first furrow.

The team used laser beams to illuminate these marks. As the surface of the paper is uneven, the light reflects at slightly different angles depending on where it strikes the furrows. This makes the light rays interfere, forming patterns of light and dark on a light-sensitive device held above the paper. The information is relayed to a computer, which constructs a holographic image of the signature.

The system can reveal characteristic patterns for signatures that have been made with ballpoint pens, fountain pens or felt-tipped pens on normal paper, cardboard or even carbon paper.

To test their system, the scientists used a database of 126 letters, each written by a different author. In almost 90% of the cases they tested, the author of a particular letter could be identified by comparing details of how their pen strokes crossed with a set of verified writing samples. For ballpoint pens on normal paper, the success rate was 100%.

The scientists say that the system could also be used to study ancient documents that may be too precious to analyse in other, potentially damaging ways. The equipment is rugged and portable, and could be taken into museums to do analyses on site. "The technique does not damage the sample," explains Schirripa Spagnolo.